The present invention relates to a machine for micro-filtering solid particles suspended in a liquid stream, more particularly water discharged from civil or industrial drainage.
It is known that drainage of civil or industrial waste water is governed by increasingly stricter laws. The European regulations presently provide that the micro-particles present in waste water should not exceed 35 mg per liter.
It is to be noted that the previous regulations, at least in force for industrial plants, allowed a concentration of micro-particles up to 80 mg per liter. Therefore there is now the need to lower the level of micro-particles from 80 mg per liter up to now allowed, to 35 mg per liter as now ruled.
In order to reach said results according to the prior art there are purification plants with sand filters, but these filters however have very big dimensions. Moreover the sand filters are limited because filtration is not continuous since said filters must be regenerated after a predetermined number of operation hours.
Another solution of the problem consists in using machines with a drum provided with filtering surfaces: the waste water to be treated is fed inside the drum and passes through the drum filtering surface. These filtering surfaces are periodically washed to avoid clogging.
One of the problems related to this filtering technique is that the machines are very big and cumbersome and their operation is expensive.
Another simpler and more efficient method to carry out filtering mainly of fibrous solids suspended in water, consists in using machines provided with filters comprising opposed rotary discs, in which water is introduced centrally in a direction almost tangent to the surface of the filtering discs and the combined action of disc rotation with water flow with a direction almost tangent to said disc, generates a force in a direction tangent to the disc and a direction normal to it. The normal component causes water to travel through the filter and carries out filtration while the tangential component is ineffective. During this filtering process a layer of fibers is formed and deposited adhering to the filtering surface, so that the fibers partially occlude the holes to which water is being filtered.
The bulk of fibers gradually formed in the filtration chamber carries out a sliding action on the surface of the filters causing the filtering holes to be self-cleaned. Obviously this self-cleaning action does not reach a perfect level. In other words these kinds of machines can operate in the above mentioned way only if they have sufficiently or relatively big holes so that the filtered water cannot have a contents of micro-particles with size less than 50 or 40 mxcexc, that is not admitted with the present regulations.
The object of the present invention is to overcome the limits of the above described known technique.
More particularly the object is to make a machine carrying out a continuous micro-filtration with considerable water flow so as to result in a filtered water having a contents of micro-particles equal or less than 20 mg per liter.
Another intended object is to provide a machine adapted to process considerable volumes of water without requiring too big dimensions.
Another object to be obtained is to provide a machine which is relatively inexpensive and highly reliable in operation.
The above mentioned objects and others that will be better indicated hereinafter are attained by the machine of the invention that according to the wording of the first claim comprises:
a feeding chamber for the liquid to be treated communicating with
one or more treating chambers, each of said chambers being defined by a couple of coaxial filtering discs rotating around their own axis arranged horizontally and normal to the direction of the water flow to be treated, and by at least a partially cylindrical wall defining together with the rims of said couple of discs, a hydraulic sealing coupling so as to enclose the liquid treating space to comprise at least the rotation axis;
conveyance chambers for the treated liquid provided with bottom outlets communicating with
at least a collection chamber for the liquid coming from said conveyance chambers, provided with discharge pipes for said liquid;
means for transmitting the rotary motion to said filtering discs,
wherein said micro-filtration machine is characterized in that each disc of said couples of discs is provided with one or more washing ramps arranged outside said treating chamber and provided with nozzles throwing water against said discs at the portion of said discs emerging from said treating chambers, the washing water being collected by at least one conveying channel for the liquid and the particles collected by said washing action, said channel being arranged between said couples of discs above the treating chamber and ending with a duct delivering the collected liquid to the purification plant arranged upstream said machine.
Advantageously the micro-filtration machine of the invention keeps the advantages of the known machines with couples of filtering discs allowing that the flow of water with suspended particles having a direction almost tangent to the discs, to intercept the disc holes according to a force component normal to the disc surface. The result of such combined action is that the disc hole is operatively smaller than the actual hole made in the disc.
The machine of the invention carries out its best performance when used downstream a water purification plant and more particularly downstream a conventional machine with filtering discs for instance of the kind disclose in European Patent 0 388 709 in the name of the same applicant. The substantial difference between the machine disclosed in European Patent EP 0 388 709 and the machine of the present invention consists in that in the machine of the present invention the self-cleaning action of the filter on the disc surface cannot be kept because filtering surfaces must be adopted with still smaller holes to obtain the micro-filtration required for lowering the contents of solids in the filtered liquid to at least 20 mxcexc or 20 mg per liter or even less.
Therefore it is clear that the filtering disc must be constantly cleaned from the filtered material so as to avoid clogging of the filtering holes.
Another advantage of the machine of the invention consists of the possibility of treating high rates of flow to be treated without needing a proportional increase of size and overall dimensions of the machine. This result is due to the effect created in the treating chamber by the hydrostatic head of the liquid to be treated. More particularly the increase of the diameter of the filtering discs and the consequent increase of the hydrostatic head generates a greater pressure on the surface of the filtering discs that in this way succeed to treat rates of liquid flow higher than the simple proportional increase of the filtering surface due to the increase of the diameter.